Many blood fluid removal systems, such as hemodialysis systems, ultrafiltration systems, hemodiafiltration systems, and the like, include components whose performance may deteriorate over time. For example, such systems often employ a medium for removal of waste products or fluid from the blood, which medium can foul and perform less efficiently over time. Such medium may include a sorbent, a filter, or the like. Over time, the medium tends to accumulate deposited proteins, blood cells and other material, which can interfere with the performance of the medium. It would be desirable to monitor the performance of the medium or other components to know when action should be taken in response to deteriorating medium, or other component, performance.